Look at the double impact basin! I can almost imagine a binary asteroid doing that, with the right hand basin being smacked by the leading rock of the pair, followed by the other one on the left… but I can’t quite convince myself. Still. Yowza.

Lousy sense of scale? To give you footing, Iapetus is about 1400 km across (900 miles), roughly the length of California. Those impact basins are freakin’ huge.

The impactors would have been far smaller than the basins, so the actual momentum transfer would have been small compared to the moon’s total momentum. On the other hand, the ejected material that left the moon would have imparted a rocket effect on it… this might make an interesting back-of-the-envelope calculation. Next time I’m bored with some time I’ll pencil it through.

I wonder if there’s a bulge opposite the large impact crater in the lower left quadrant? I’m thinking analogically of Mons Olympus,,,like, how do you get a volcanic cone 39,000 feet tall from merely geologic processes?

As I was picking up The Little Astronomer from school, I realized I blew it: the rocket effect won’t help here. The energy of the ejecta from Iapetus would all come from the kinetic energy of the impactor, so the momentum transfer of the impactor is all that matters.

The crater on lower light seems to be older because it is less deep and seems to be covered by the other impact crater. A double impact would create a different feature as the impacts would have been simultaneous.

You can see yet another giant crater near the top of the image. But that’s not all, of course. There are even bigger basins on the dark side. The biggest has a diameter of 500 km; the “Death Star” moon Mimas would easily fit there.

The large number of giant basins doesn’t have to mean anything except that the surface of Iapetus is extremely old. And also porous, otherwise they wold have flattened ages ago as has happened on Tethys for example.

Although those impact craters are relatively fascinating, we have plenty of them around the Solar System; what is freakingly odd about Iapetus is its waist-band that challenges the largest mountains in the Solar System in height, and even more that the duotone color which has been an enigma since the discovery of the moon in the 17th century. Was Iapetus originally white, and then was coated with dark material, or was it originally dark like many of the outer Solar System bodies and then was coated with snow? The first is–at least until the flyby–considered to be the case. Some hires images from the flyby suggest otherwise. Whatever the truth is, the coloring must have been a very complicated process. Any theory must take account that the border between dark and light regions is puzzlingly sharp even at highest resolutions.

JIm P,. 11 out of the past 15 posts have been about astronomy and/or skepticism, not counting the posts on religion. I have sampled the numbers many times, and the vast majority of my posts are about astronomy.

Just so’s you know.

Kullat, a binary asteroid might hit at two different times depending on the separation, impact speed, and so on. Tidal forces with Saturn could separate the two, for example. The basins are both almost exactly the same size, and that is very suspicious.

Forgive my enormous ignorance, but by “double impact basin” are you referring to the feature that occupies the entire lower left quadrant of this image? The contours of the two basins seem very congruent, reminiscent of the way the East coast of South America fits into the West coast of Africa. Is it possible that there is some other, more direct relationship between these two features?

Between this, the rings, the new questions that the Huygens probe brought up, and that hexagon weather pattern the north pole, there should be a lot more probes at Saturn or on the way to it. What a strange place!

Jeffrey Cornishon wrote:
“… Look at the other moons in Saturnâ€™s retinue that have larger craters, but not terribly eccentric orbits.”

– And assuming that we know anything at all?
Sorry, but there’s an a priori argument about.
This ‘impact’ is a postulate – an added event-
but it is not evident.
What about theories that deal with evidence alone?
We may now have a science to win these pictures, but
“astronomy” simply does not have a science to interpret them consistently.

Even the local universe is indeed far larger than our philospphy.
We might yet understand, should questioning ever become
more valuable than ‘knowing’.

Hmmm, in the pictures Ive seen of Iapetus I can only see the walnut bulge on the dark leading side of the moon. Further the dark smudging seems to be centered on the bulge. Perhaps the dark material on the surface IS swept up dust from wherever and the bulge isn’t an upwelling of the crust but simply a build up of dust, as if most of the dark dust Iapetus swept up were concentrated on a thin plane. Again maybe I’m just not seeing it clearly but i don’t see a walnut bulge on the light side.

I remember as a boy reading speculations as to what Iapetus might be like with its then barely glimpsed bright and dark sides, illustrated by Chesley Bonestell’s imaginative yet equally speculative artwork, and now we have this actual image. Nature is always stranger than we can imagine.

hilarleo said: Sorry, but thereâ€™s an a priori argument about.
This â€˜impactâ€™ is a postulate – an added event-
but it is not evident.

That really depends on how you want to define “evident.” But since the features display a multitude of details that would be present from an impact, and since the physics support such details, and since every body of our solar system sports impact basins, and since no one has shown distinct evidence that could not come about from impacts, the evidence all points to impacts. That’s how most people define “evident.”

This is not to say that anyone has settled the matter once and for all. If anything new is observed, it can change the idea that these are impact basins. Happens all the time, and BA has pointed them out on more than one occasion. But until then, the prevailing theory is used, because the evidence supports it.

What about theories that deal with evidence alone?

Okay, three responses all at once, aren’t you lucky?

1) What theories would those be? (I say this is all seriousness – what theories exist that support the idea that these are not impact basins?)

2) Are you differentiating those theories in some way from the one that says these are impact basins? And in what way?

3) You should probably know that a “theory that deals with evidence alone” is self-contradictory. Theories postulate possible causes, connections among all the facts and the observations we have made regarding physics. It’s not even possible to deal with “evidence” on a distinct basis. You can call anything into question, because nothing can ever be proven beyond a shadow of a doubt: Show me a rock, and I tell you to prove that it is not simply your senses telling you it’s a rock. But science works on the preponderance of evidence. Hand that rock around to enough people, and they’ll describe the same features. Is this truth? No. But it works amazingly well for our world.

We may now have a science to win these pictures, but
â€œastronomyâ€ simply does not have a science to interpret them consistently.

Um, yes, actually, it does. And what makes this statement most ironic is what you say immediately afterwards:

Even the local universe is indeed far larger than our philospphy.
We might yet understand, should questioning ever become
more valuable than â€˜knowingâ€™.

One of the vaguest statements I’ve seen in a while, but what it seems to be saying is that “we” (and I’m guessing scientists are largely targeted with this pronoun) would be better served by not assuming absolute knowledge. Correct me if I’m wrong (or state your case a lot clearer).

Again, there is no assumption of absolute knowledge, and if you believe there is, you certainly haven’t read a handful of scientific literature, or even press-releases on science subjects. Because every last publication or statement is not prefaced with, “Based only on what we have interpreted as our observations,” or something similar, you think that, “Look at these impact basins!” is actually saying, “They are definitely impact basins, they cannot be anything else, we have to have proven this before calling them impact basins.” Not in the least. They show distinct details of impact basins, so, for convenience’s sake, we call them that. Makes the conversations much shorter and not ridiculously torturous.

And (providing that I am reading your statement correctly), you should probably have recognized that, by itself, such a standpoint defines an a priori argument pretty well

Now, go read your last sentence again, and tell me in what way you are differentiating “understand” from your own use of “knowing.” Did you really just say, “We might know if we don’t worry about knowing”? Is there a purpose to such a philosophy? It basically says to deny all knowledge.

You are doing the world of science, and all those who even have an interest in it, a huge disservice by suggesting it should start questioning things. That pretty much defines science, and we didn’t really start to discover things until we began to question them (rather than following the dogma of the culture). Once we did that, the storehouse of our knowledge increased enormously. In the vast majority of cases, it did so by working with knowledge gained by those who came before. In other words, knowledge begets knowledge.

You yourself might be better served by examining the evidence (in this case, of the scientific method and how it’s used) rather than being proud of the chip on your shoulder. Using quotes around the word “astronomy” is a dead giveaway.

Speaking of Iapetus…on the Cassini-Huygens webpage there is a nice video greeting from Arthur C. Clarke to mark the Iapetus flyby. Here’s hoping that the flyby will yield information more wonderful and stranger than even he can imagine.

At first glance I was reminded of an ice-cream bombe, with a dusting of chocolate powder, then the mystery deepened..

To me, the dark areas don’t look like dust ejecta, but a material that is underlying a layer of water ice.
For instance, there are several dark craters with a central patch of ice and all the dark areas I can see have well defined edges.

It’s been suggested that polymerisation of ethane and methane by ultra violet radiation might be responsible the dark material.
Could it be that some kind of differential sublimation of ice is going occuring, which exposes it?

If so, why is it so pronounced one side of the moon?
(This been observed since Cassini’s observations (the person that is) in the 17th C)

The right hand basin looks older than the left hand, geologically so. However, both are at suitable latitudes to deflect Iapetus’ orbit to an inclination, which we observe. Impact orbital modification should result in a more eccentric orbit, though.

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Wayne, I think it would take a lot bigger impactor to nudge the orbit that much than what is seen here. Whatever hit there must have been only a fraction of Iapetus’ mass and so it couldn’t have imparted too much momentum.

The left hand crater seems deeper and sharper than the right, though they both show similar surface texture and smaller craters.

Harold said:
> Forgive my enormous ignorance, but by â€œdouble impact basinâ€ are you referring to the feature that occupies the entire lower left quadrant of this image? The contours of the two basins seem very congruent, reminiscent of the way the East coast of South America fits into the West coast of Africa. Is it possible that there is some other, more direct relationship between these two features?

What other relationship do you propose? There does appear to be some similarity of the shapes.

Mena :”Between this, the rings, the new questions that the Huygens probe brought up, and that hexagon weather pattern the north pole, there should be a lot more probes at Saturn or on the way to it. What a strange place!”

You know, I completely agree. If asked if Jupiter or Saturn would have the oddest features, Jupiter would have gotten my vote. They’re both pretty interesting, but Saturn is taking the cake.

Irishman, I was thinking either of some sort of slumping action of material from the “real” crater to the “ghost” crater to the lower right, or a steady march of glaciers (or glacier analogues made of whatever the white material is – does spectroscopic analysis confirm that the white stuff is water ice?), or – and this would be weird – could we seeing a massive displacement of material – a roundish chip that has popped out of the surface and displaced several tens of miles, so that we are seeing the congruence of “part” and “counterpart”? There are probably several other weird processes that could explain the remarkable congruence. I guess I’m just wondering why the edges in a double impact basin would be so very similar, and I’m wondering if the answer could be that this isn’t a double impact basin at all.

BA, in a couple of comments earlier in the thread, you talk about momentum transfer to Iapetus from the impactors that created a pair of large craters. At first you say that we need to consider momentum from matter ejected on impact as well as the momentum of the impactor. Then you change your mind and decide that we only need to consider the momentum of the impactor.

I’m pretty sure that you were right the first time. (But even with the ejected matter considered it was a pretty small nudge to Iapetus.) So please, go ahead with that back of envelope calculation and let us know what you find.

Yeah, I agree with Michael Welford. The momentum change of Iapetus equals the momentum change of the impactor, so if some of the impactor bounces off, the change is bigger. Still, I’m betting it’s a small correction to a number that’s already small.

As for whether the two craters are related, when the sample space is the whole ancient solid surface of the Solar System, getting two overlapping craters of similar size *somewhere* by chance doesn’t seem that much of a stretch. It’s going to seem dramatic at the moment of discovery, sure.

For Irishman: Haleakala is the mountain which comprises the western lobe of the island of Maui in the Hawaiian Islands. Near its 10,000-foot (3000-meter) summit there are astronomy facilities and perhaps BA has some association with those.

Haleakala is also known for having a spectacular sunrise view which tourists have been watching for over a century (Mark Twain wrote of his experiences in “The Innocents Abroad”). I have seen it myself and it is worth the trouble. I got to see the edge of the earth’s shadow move through the faint haze in the sky, and I swear that I felt a “thump” when the shadow’s edge reached me and the limb of the sun crept over the horizon. Looking toward the west I got to see the shadow of Haleakala seem to converge to the other horizon. As a bonus while waiting for the sunrise, I got to see Alpha Centuri and Omega Centuri for the first time (although I didn’t have a pair of binocular with me–rats!).

If anyone reading this would like to see this phenomenon when they visit Maui, know that despite being in Hawaii the combination of the altitude and the wind-chill factor make it mandatory that you bring some wind protection if not actual cool weather gear. When I was there I wouldn’t be surprised if the pre-dawn temperature was around 50 degrees F and wind-chilled to under 40 degrees F (5 degrees Celsius). You also have to start your trip up the mountain quite early–2 or 3 am–or you won’t find a parking spot in the small lots near the summit, and you really don’t want to have too long a hike at the 10,000-foot altitude without some previous training.

Michael Welford: I think I was right in the second post. The rocket effect comes from the ejecta being propelled off the surface of Iapetus, right? Where does that energy come from? Right, the impactor. So all the energy imparted onto Iapetus comes from the impactor, and that’s all we need to deal with. Whether that energy goes into direct impact with Iapetus or into splashing some of it away doesn’t matter; only the total energy (or momentum, if you wanna do it that way) matters.

Arthur Maruyama, the phrase itself comes from the old TV show “L.A. Law”, where it is uttered by a character while on vacation in Hawaii. I won’t recount the circumstances here (very NSFW, but hilarious) but the phrase cracked me up, which is why I use it. It in fact has nothing at all to do with the observatory, ironically enough!

Momentum is conserved, so the total momentum of the system = (mV of Iapetus + mV of the impactor + mV of all material blasted off the moon and lost) = the same after the collision as before.
Momentum is conserved but kinetic energy is not. The size of the crater is related, roughly, to the relative kinetic energy of the impactor = 1/2 mV^2.
A very high relative-velocity impactor of low mass would blast a huge crater while imparting much less change of momentum to the Iapetus/impactor system than would a very massive impactor moving at much slower relative speed while blasting a similar sized crater.

That amazing, half-a-world-wide “wall” trumps all the other strangeness of this moon, in my opinion.

“…only the total energy (or momentum, if you wanna do it that way) matters.”
Kinetic energy is not the same thing as momentum! I hope you’re being casual here.

Itâ€™s your site, do what you want with it, but the lack of science is making my visits less frequent.”

BA, you have a strong voice thanks to your website and your reputation.
Please continue to use your strong voice to resist the ever-escalating war against reason that is being relentlessly waged by a handful of hyper-wealthy religious fanatics (and the hyper-wealthy who pretend to be religious).

I am an American.
I want my nation to remain the bastion of sanity and reason that it was created to be by our Founding Fathers, not the Puritan witch-hanging horror that a few among us are manipulating the rest of us to become.
Please keep defending us! Defend reason!

BA:I’m going to stand my ground on the impactor scenario discussed above. Following bliznos line of argument, with a sufficiently low mass impactor we can make the kinetic energy arbitrarily high while making the impactors momentum arbitraily low. So in theory we can put as much energy and momentum into the ejecta as we want. In theory.

Whether this will be true with realistic speeds I will leave to professionals. ( That would be you. )

“BA, you have a strong voice thanks to your website and your reputation.
Please continue to use your strong voice to resist the ever-escalating war against reason that is being relentlessly waged by a handful of hyper-wealthy religious fanatics (and the hyper-wealthy who pretend to be religious).

I am an American.
I want my nation to remain the bastion of sanity and reason that it was created to be by our Founding Fathers, not the Puritan witch-hanging horror that a few among us are manipulating the rest of us to become.
Please keep defending us! Defend reason!

Defend science.”

Hear! Hear!

That goes I think for all rational people.

All too sadly, unless the nuttiness of religious nuts is pointed out clearly all too many people who don’t think clearly enough tend to fall for the religious nutters nuttiness. Well nuts to that!

If we are to have a positive future, the backward thinking, evil, regressive, intolerant, anti-choice (all choice too – not just women’s) , plain outright wrong religious nuts _need_ to be stopped from controlling everyone’s lives.

Americans (USA-ites, that is Cubans, Canadians, Peruvians, etc .. not included here) generally, seem unfathomably anti-intelligence and pro-Christian extremist which is utterly self-destructive and harmful for the entire planet not to mention a tragic waste of a nation with so much potential as well.

This may be a dead thread at this point, but I want to put in my two cents regarding the Iapetus coloring. I agree with Alex. Upon close examination of the photos at planetary.org, it seems (strongly, to me) that there is not a “dusting” of black stuff, but a dusting of white stuff over a fundamentally dark Iapetus.

By making the black part of Iapetus correctly exposed, and overexposing the light part, it can be seen that there are dark colored craters partly covered by white. It wouldn’t make sense to have a white base that was partly covered by a half crater.

I say the dark area of Iapetus is where the “snow” (or whatever it is) has been removed (or didn’t accumulate); perhaps by sublimation or some such process. If the dark side of Iapetus faces Saturn, maybe the Saturnshine or other radiation from Saturn has caused the removal of the white stuff from that side.

re:PsyberDave:
I’m glad someone chirped up on this point, because I thought I’d committed some horrible faux-pas and was being relegated to the crackpot category.

I’ve studied the blown up image of Iapetus in detail and I can’t find anything that convinces me that the icy material isn’t overlying the dark matt material. If someone can convince me otherwise, I’d be happy to accept their arguments.

To me, the lighter areas, look very much like what happens when snow begins to melt. ie. patches of hardened icy-stuff persist and gradually pock-marks and holes develop.

If there is some dynamic melting process going on, couldn’t it be solved rather simply by observing the moon in detail over a few weeks?

Since there is evidence that it’s been something like this for centuries, I like the idea that Saturn’s causing it. Perhaps something to do with magnetic fields, or maybe it just radiates enough e-m? Isn’t it like Jupiter, a planet which emits more than it receive?

There’s a rather convincing case to be made regarding the black-on-white scenario. When Cassini flew over Cassini Regio, it saw NUMEROUS fresh impact craters which show excavated bright ice. To this date, I haven’t seen anything resembling an impact crater excavating dark stuff over white ice in Roncevaux Terra. To me this is a very convincing argument that the dark stuff is a pretty thin coating on an otherwise icy moon (just like all other Saturnian moons are).

Certainly looks like ice at higher elevations when you look at the boundary. Questions that occur to me:

Would the dark area absorb sufficient energy to keep itself ice-free?

Does the dark side of Iapetus face toward or away from Saturn? (Effect of more incident sunlight?)

Could Iapetus have enough heat generated internally (Saturn squeezing it must cause some) to keep the surface partially ice-free? Perhaps the internal structure is non-uniform, so the energy output is also non-uniform. (Would mapping Iapetus for gravity distribution at surface confirm the internal structure?)

Quite a neat wee moon.

I am more interested in what further surprises Europa might have in store, though. A water-ice covered moon with an oxygen atmosphere (well, OK, tenuous atmosphere). Hmm.

Gordan, to me it seems like PsyberDave is not talking about the shading of the terrain, but rather the underlying texture evident in the images. The dark filtered left image shows texture in the white areas, the exposed right image shows texture in the dark areas. Trying to map between the two images to see where craters in one image overlap the other image and look for terrain indicators is tough. I can see places where the craters are evident in both images.

I find it difficult to discern between ice on a rocky surface and “soot”* on an icy surface. I could be persueded either way from these images, which does not provide conclusive evidence. Now the comments about fresh craters and the exposed white does seem more suggestive.

*I don’t think it is soot, but just provoking the imagery of soot on ice.

I don’t know if you’ve covered this yet, or are planning to, but here’s an update on the latest thinking about mysterious Iapetus:-

“NASA scientists are on the trail of Iapetus’ mysterious dark side, which seems to be home to a bizarre “runaway” process that is transporting vaporized water ice from the dark areas to the white areas of the Saturnian moon.

This “thermal segregation” model may explain many details of the moon’s strange and dramatically two-toned appearance, which have been revealed exquisitely in images collected during a recent close flyby of Iapetus by NASA’s Cassini spacecraft.

Infrared observations from the flyby confirm that the dark material is warm enough (approximately minus 230 degrees Fahrenheit or 127 Kelvin) for very slow release of water vapor from water ice, and this process is probably a major factor in determining the distinct brightness boundaries.

“The side of Iapetus that faces forward in its orbit around Saturn is being darkened by some mysterious process,” said John Spencer, Cassini scientist with the composite infrared spectrometer team from the Southwest Research Institute, Boulder, Colo. ”